Using Blender as a video editor can I only add an audio track without recoding a video track?
No, it can't.

Blender only sends its render output to ffmpeg internally which means it's being encoded (again when using decoded footage in the VSE). You can use Blender as a Frameserver as well, but that won't help you out either.

Instead you should use Avidemux or mkvtoolnix to accomplish your goal.



Edit: As pointed out by Troy James Sobotka in the comment below, using FFMPEG directly would be a solution:

ffmpeg -i video.avi -i audio.mp3 -codec copy -shortest output.avi